Metal tubes of heavy wall are difficult to flare so that it is not practical to utilize such heavy wall tubes with conventional flared type tube couplings such as shown in U.S. Pat. No. 3,265,413. As a consequence, such tubes are either welded into suitable couplings or are used in conjunction with flareless type couplings such as shown in U.S. Pat. No. 3,499,671 wherein a sleeve is radially contracted during assembly of the coupling parts into gripping and sealing engagement with the unflared tube. However, such flareless coupling joints have several disadvantages, particularly when used with heavy wall metal tubes.
One disadvantage is that the sleeve may not secure sufficient grip upon the tube to withstand the high pressures for which the heavy wall tube is intended. Another disadvantage is that if the flareless coupling is insufficiently tightened the sleeve may not be sufficiently contracted for gripping and sealing the tube and the joint may leak or the tube may blow out of the coupling with disastrous consequences.